1. Field of the Invention
The present invention relates to a voltage regulator for generating a constant voltage based on an input voltage.
2. Description of the Related Arts
A mobile electronic device such as a mobile telephone is generally operated by power supplied from a rechargeable battery. An output voltage of the rechargeable battery changes corresponding to a charging state. In order to stably operate the mobile electric device, it is necessary to apply a constant voltage to the mobile electric device. Therefore, the mobile electric device includes a voltage regulator for generating a constant voltage without depending on the output voltage of the rechargeable battery. For circuit protection, the voltage regulator includes a rush current limiting circuit for limiting a rush current of an output stage transistor.
Hereinafter, a conventional voltage regulator including a rush current limiting circuit will be described. FIG. 4 is a schematic circuit diagram showing the conventional voltage regulator.
The conventional voltage regulator includes an amplifying circuit 25 for comparing a reference voltage with a dividing voltage obtained by division of an output voltage, an output stage transistor T23 through which a drain current corresponding to an output voltage of the amplifying circuit 25 flows, a testing transistor T24, a current limiting circuit 20 for controlling a gate voltage to the transistor T23 and the transistor T24 based on the drain current of the transistor T24, a switch circuit 30 for switching between input paths of the drain current flowing from the transistor T24 to the current limiting circuit 20, an on/off circuit 26 for controlling an on/off operation of the voltage regulator, and a counter circuit 27 for measuring an elapsed time from the time when the voltage regulator is turned on. The on/off circuit 26, the counter circuit 27, and the current limiting circuit 20 are collectively referred to as a rush current limiting circuit.
The current limiting circuit 20 includes a first output current limiting circuit 21 and a second output current limiting circuit 22. The first output current limiting circuit 21 detects a first current limit value to limit the drain current of the transistor T23. The second output current limiting circuit 22 detects a second current limit value higher than the first current limit value to limit the drain current of the transistor T23. The counter circuit 27 controls the switch circuit 30 based on the elapsed time. The switch circuit 30 connects the transistor T24 with the first output current limiting circuit 21 until a predetermined elapsed time has elapsed. After the predetermined elapsed time has elapsed, the switch circuit 30 connects the transistor T24 with the second output current limiting circuit 22.
The operation of the conventional voltage regulator as described above will be described.
When the voltage regulator is turned on, the on/off circuit 26 causes the amplifying circuit 25 to start activation and causes the counter circuit 27 to start counting. In order to rapidly charge an external capacitor (not shown) connected with an output voltage terminal, an excessive drain current (rush current) flows through the transistor T23. A drain current proportional to the rush current, of the transistor T24 flows into the current limiting circuit 20. The switch circuit 30 selects the first output current limiting circuit 21 based on an output of the counter circuit 27. When the drain current becomes equal to or larger than the first current limit value, the first output current limiting circuit 21 controls the gate voltage to the transistor T23 and the transistor T24 to reduce the drain currents thereof. After a predetermined time has elapsed from the time when the voltage regulator is turned on, the switch circuit 30 selects the second output current limiting circuit 22 based on an output of the counter circuit 27 (see, for example, JP 2003-271251 A).
When an input voltage slowly increases, it is unnecessary to limit the drain current of the output stage transistor by the voltage regulator. However, until the predetermined elapsed time has elapsed from the time when the conventional voltage regulator is turned on, the first output current limiting circuit 21 whose current limit value is low limits the drain current of the output stage transistor T23. Therefore, because of unnecessary limitation given to the drain current, a current for charging the external capacitor connected with the output voltage terminal reduces, with the result that a rise time of the output voltage of the voltage regulator becomes longer.